Cytokinesis is the final stage of the cell cycle when the cytoplasm is cleaved into two parts. Proper timing and location of the cleavage is essential for cell division to result in two normal daughter cells. In animal, fungal and amoeboid cells cytokinesis involves constriction of an actomyosin contractile ring. The contractile ring of fissio yeast Schizosaccharomyces pombe is particularly amenable to quantitative modeling, as the ring proteins are particularly well characterized. However the mechanisms that generate ring tension and constrict the ring are not well understood. From the first period of support, we have developed a working computer simulation framework for the fission yeast contractile ring, the first working method to measure ring tension experimentally, and a model of the septation process that accompanies ring constriction. The first Aim of the proposed research is to build a fully dynamic 3D simulation to capture the ultrastructure of the ring. We will implement detailed organizations of the two myosin-II isoforms in the fission yeast ring, Myo2 and Myp2, together with actin, the actin nucleator formin, actin crosslinkers and other key components. The simulations will test hypothesized organizations by calculating ring tension and component motions for direct comparison with experimental values. Each organization will be tested for structural instabilities, particularly in simulations of rings with mutations in ring proteins. The model will also simulate detailed component motions in rings that slide in yeast protoplasts, cells whose walls have been removed, for comparison with experimentally imaged sliding rings. The second Aim is to develop an integrated model of fission yeast ring constriction and septation. In the coupled simulation scheme the septum shape dictates to the ring the evolving surface to which it is attached, while spatiotemporally varying ring tension feeds back to septum growth. Tensions, organizational stability, septum morphology and other outputs of the integrated model will be compared to experiment. The third Aim is to model the behavior of isolated fission yeast cytokinetic rings, as studied in experiments using permeabilized yeast protoplasts. In these circumstances normal component turnover is blocked, as new components do not bind the ring, and rings often become partially unanchored from the plasma membrane. As a result, the behavior of rings in these conditions probes the role of turnover and anchoring in the mechanisms of tension production and constriction of the ring. The ring simulations will be run without component binding, and with partially anchored rings, and the behavior compared to experiments.